1. Field of the Invention
The present invention relates to a surface acoustic wave device which comprises a surface acoustic wave element that is formed with an inter digital transducer electrode (hereinafter referred to as “IDT electrode”), a connector electrode and a periphery sealing electrode and joined to a base substrate through a solder bump component and a solder sealing component, and a method for manufacturing the same.
2. Description of the Related Art
Conventional surface acoustic wave devices include a monocrystal piezoelectric substrate such as a lithium tantalate substrate or a lithium niobate substrate with one principal surface formed with an IDT electrode, in which a connector electrode for providing signals to and receiving signals from the IDT electrode is formed on the same principal surface.
These IDT electrode and the connector electrode are formed on the piezoelectric substrate with use of aluminum or the like by a thin film forming method.
The base substrate comprises, for example, alumina ceramics or the like, and an electrode for connection to the element connected to the connector electrode of the surface acoustic wave element is disposed on the principal surface of the base substrate.
There has been a known structure using solder bumps for joining such a connector electrode of the surface acoustic wave element to the electrode for connection to the element of the base substrate.
In this case, Pb/Sn (for example, 95Pb/5Sn) is generally used for the solder for joining the surface acoustic wave element and the base substrate together.
In the structure in which solder bumps are used, unlike the case where gold bumps are used for connection of the surface acoustic wave element, it is not necessary to apply ultrasonic vibration to the surface acoustic wave element, so that physical damage to the surface acoustic wave element can be significantly suppressed. When a heat treatment such as a solder reflow treatment is carried out, the lead component in the solder has the function to absorb stress caused by the difference in thermal expansion coefficient between the substrate and the surface acoustic wave element. This is because the lead component contained in the solder is a relatively soft material, and even if stress is generated due to the difference in thermal expansion coefficient, it can be mitigated by the soft lead. That is, using a solder containing lead makes it unnecessary to strictly consider the thermal expansion coefficients of the base substrate and the surface acoustic wave element, and allows for relatively inexpensive, easy joining.
However, since lead-containing solders used for joining components contribute to deterioration of the environment, the use thereof is to be restricted.
In addition, because of the weak yield stress of the solder materials containing great amounts of lead, when thermal stress is generated due to the difference in thermal expansion coefficient between the surface acoustic wave element and the substrate, great amounts of plastic strain and creep strain are caused. As far as the performance during the soldering operation is concerned, such materials seem to be able to absorb the thermal stress as described above. However, when they are subjected to a temperature cycle test assuming an actual long duration of use, stress due to the difference in thermal expansion coefficient between the surface acoustic wave element and the base substrate acts intensely on the solder joint. The stress repeatedly acts on the solder joint causing metal fatigue in that section, finally breaking the joint.
In addition, in the foregoing surface acoustic wave device, since heat from the solder reflow or the like also propagates to the interior of the surface acoustic wave device when it is mounted on the motherboard, the soldering material that joins the surface acoustic wave element and the base substrate together is also subjected to heat. As a result, the soldering material is expanded by the heat and the volume thereof is greatly expanded particularly during remelting.
The side surfaces of the surface acoustic wave device are covered with an outer covering resin layer which is firmly fixed to the base substrate. This causes the soldering material that has been expanded in volume due to the remelting to flow toward the interior of the gap between the surface acoustic wave element and the base substrate. As a result, short circuit would occur in the connector electrode of the surface acoustic wave element.
In addition, due to the use of a pyroelectric component for the surface acoustic wave element of the surface acoustic wave device, sparks are generated during the reflow treatment between electrode fingers of the comb-shaped IDT electrode engaged with one another on the piezoelectric substrate, causing pyroelectric breakdown to occur in the IDT electrode. The generation of sparks becomes prominent when the speed of temperature increase during the reflow treatment is high. For this reason, pyroelectric breakdown has been conventionally prevented from occurring by lowering the speed of temperature increase. However, in this way, the reflow treatment takes too long time, which makes it difficult to manufacture surface acoustic wave devices with high efficiency.
When the reflow treatment is performed in a chamber with a nitrogen atmosphere, the surface acoustic wave element must be kept stably mounted on the base substrate. This is because misalignment cannot be corrected after the base substrate and the surface acoustic wave device are introduced into the chamber because of the nitrogen atmosphere of the chamber. To deal with this problem, conventionally, a flux is contained in the solder bumps and the viscosity of the flux in the solder bumps is utilized for stable temporary fixing of the surface acoustic wave element to the base substrate.
However, if the reflow treatment is carried out in such a condition, the flux component scatters inside the gap surrounded by the solder sealing component and adheres to the surface of the surface acoustic wave element, leading to deterioration of the properties.
In addition, when solder bumps containing or provided with a flux are used, the solder bumps close the gap formed between the base substrate and the surface acoustic wave element before the reflow treatment in the chamber with a nitrogen atmosphere. For this reason, even if the pressure inside the chamber is reduced to bring the chamber into a nitrogen atmosphere, it is difficult to reliably bring the gap region formed between the surface acoustic wave element and the base substrate into a nitrogen atmosphere.
It is an object of the present invention to provide a surface acoustic wave device in which a base substrate is joined to a surface acoustic wave element employing a piezoelectric substrate, which allows very stable joining to be accomplished resisting thermal stress due to a difference in thermal expansion coefficient and permits stable connection to be maintained for a long duration of time.
It is another object of the present invention to provide a surface acoustic wave device in which electrical short circuit in the surface acoustic wave element due to remelting of the inside solder at the time of mounting on a motherboard or the like is prevented from occurring.
It is still another object of the present invention to provide a method for manufacturing a surface acoustic wave device with high manufacturing efficiency that is able to preclude breakdown of the surface acoustic wave element due to sparks and deterioration of the properties, and to reliably and simply bring the inside of the gap into a nitrogen atmosphere.